Swing plane is simply the angle that you swing the club head around you at and this angle is absolutely crucial as it influences how well the club head will connect with the golf ball, the direction that the golf ball will fly in and also the distance that the ball will be hit.
If you address the golf ball with a very upright posture, you will tend to have a very upright golf swing. If you place an alignment pole into the ground on the right hand side of you (for right handed golfers), this will help you to understand your swing plane. Generally, place the alignment pole into the target line about three inches to the right of the golf ball and push it into the ground so that it is leaning at about a 45 degree angle on your side of the target line and at a right angle to the target line.
If your posture and spine angle are too upright, your swing plane will be too upright and this means that as you swing the golf club away from the golf ball, the club head will move too vertically up from the alignment pole. This will happen because an upright posture promotes you to use your arms to swing the club back and reduces your upper body rotation. The club will return back down towards the golf ball at a very steep angle which will result in the club head striking the top of the golf ball and producing a low golf shot. Because the arms were used to lift the golf club away from the golf ball more than the upper body rotated, the club head will not be travelling at maximum club head speed so the shot will not be struck will full club head speed and this will also reduce the distance that the golf ball travels. This will also give you directional issues and make it very difficult to strike the golf ball straight along the target line.
If you tilt too far forward with your spine during your address then you will swing the golf club on a flat plane, or below the alignment pole. This lower angled golf swing will make it difficult to attack back towards the golf ball with the club head at the correct angle and the club head will tend to strike the ground before it strikes the golf ball. The speed from the club head will be transferred into the ground rather than the ball and the shot will not travel very far. The club head will also approach the golf ball from inside the target line and this will make it very difficult to swing the club head directly along the target line which you need to do to hit a straight golf shot. You will have directional issues with your golf shots, with this swing.
Rather than tilting forward, if you curve your spine forwards from your shoulders and create a C look to your posture this will make it very difficult to rotate your upper body correctly and you will tend to swing by lifting your arms up and this will produce a very upright swing plane with the previously discussed problems.
To create the correct spine angle at address, try the following drill. Place a club down your back so that the back of your head, the middle of your shoulders and the bottom of your back all touch the club. This has your back straight and you need to maintain this as you tilt forward from your hips. You want to tilt forward enough so that your arms can dangle directly under your shoulders and this is the position to achieve to hit your golf shots from. This will allow you to rotate your upper body correctly and if you maintain this spine angle, you will swing the golf club on the correct plane helping you to achieve consistency, accuracy and length with all your golf shots.